The present invention relates to a sheet feeding device for feeding a printing sheet or copy sheet to a printing portion of a printer or copying portion of a copy machine, and more particularly, to a sheet feeding device utilizing a cassette case attachable and detachable to and from such a machine for stacking a plurality of sheets and having a function capable of controlling a pair of feed rollers for feeding the sheet located on the top of the stacked sheets so as to be brought out of contact with the surface of the sheet when the sheet is fed and the cassette is detached from the machine.
In a sheet feeding device having a cassette described above, a plurality of sheet types which differ in sheet size, sheet quality and so forth, have been housed in different cassette cases. As necessary, the cassette cases are changed to feed a desired sheet to the machine. Since this type of sheet feeding device is convenient, it has been widely used. In this cassette type, a case housing portion is provided in a machine such as a printer or copy machine. In the case housing portion, a cassette case which houses a plurality of sheets being stacked is inserted. A variety of types for sheet feeding devices having a cassette have been announced, one of which provides a pair of sheet feed rollers and separation nails desclibed below. In other words, sheet feed rollers are provided near a case housing portion. The sheet feed rollers touch the sheet located on the top of the stacked sheets and are rotatably driven, thereby forwardly feeding the sheet.
On the other hand, the cassette case is provided with a tension member for pressing the bottom side of the bottom sheet of the stacked sheets in such a manner that the top sheet is brought into contact with the sheet feed rollers. In addition, the cassette case is provided with a pair of separation nails which are located in such manner that they are touched at least from the front side and upper side to both left and right corners of the front end in a sheet feed direction of the sheet being stacked and housed in the cassette case.
When the sheet feed rollers feed the sheet, both the corners of the top sheet are bent and deformed so as to allow only the top sheet to be fed. The separation nails are supported by sheet separation portions which are movable in a direction nearly the same as that in which the paper is stacked.
The method where sheet is fed sheet by sheet using its frame is named a "paper separated and bent by nail" method.
In the paper feeder described above, when the cassette case is detached or attached, if the stacked sheet in the cassette case touches (interferes with) the sheet feed rollers in the case housing portion, some troubles such as wrinkling, bending, and deviation from a predetermined position will occur. Further, in this type of sheet feeding devices, when a sheet is sent from the sheet feed rollers to a feed mechanism sequentially located to the sheet feed rollers, the above troubles will occur if the sheet is brought into contact with the sheet feed rollers. Thus, it is necessary to prevent the sheet from touching the sheet feed rollers when the cassette case is attached and detached, until the sheet fed from the sheet feed rollers reaches the feed mechanism.
To do that, in one type of a sheet feeding device which has been devised, as shown in FIG. 1 the sheet feed rollers 106 mounted on an end of a switching arm 130 are moved between a contact position where the sheet feed rollers 106 touch the top sheet in a cassette case 102 and an escape position with a predetermined length about a support shaft 131, and the sheet feed rollers 106 are touched to the sheet only when a sheet feed operation takes place (The bottom side 104 of the bottom sheet is pressed in a vertical direction by a spring 105 as a tension member, as described before.).
In another type of a sheet feeding device, as shown in FIG. 2 a part of periphery of each sheet feed roller 206 is cut so as to form a "D" shape. In the normal state, a flat portion 206a is opposed to the top sheet of a sheet P in a cassette case 202 and a predetermined distance therefrom. In the sheet feed state, the arc portion of each "D" shaped roller 206 is touched to the top sheet of the sheet P so as to feed it (The bottom side 204 of the bottom sheet is pressed in a vertical direction by a spring 205 as a tension member, as dscribed before).
However, in the former structures, since a mechanism for movably supporting the sheet feed rollers 106 and drive member thereof are required, the structure will become complicated, resulting in increasing the cost of the production and possibility of troubles. In addition, although the sheet feed rollers 106 are touched to the top sheet whenever fed, the reliability of sheet feeding will not be satisfactory due to a problem of precision of the sheet feed position.
On the other hand, in the FIG. 2 structure, the home position of the sheet feed roller 206 (where the flat portion 206a is opposed to the top sheet) should be always controlled, resulting in increasing the cost of the production. In addition, at the arc portion of the D-shaped sheet feed roller 206, a sheet should be fed with a predetermined stroke, thereby requiring the diameters of the sheet feed rollers 206 to be large. Moreover, where two sheet feed rollers 206 on the left and right exist, if the "D" cut portions on the left and right are not equal, sheet skewing which inclinedly feeds sheet takes place.
In addition, Japanese Patent Provisional Publication No. SHO49-18336 discloses apparatus for housing a cassette in a copy machine wherein a dowel and cam plate are relatively provided in a sheet holding plate of the cassette and a cassette holder of the machine. When the cassette is attached, the dowel and the cam plate are engaged so as to push down the holding plate, thereby causing the top sheet not to touch the sheet feed rollers. When the cassette has been completely attached, the sheet touches the sheet feed rollers.
Although the above structure is simpler than those using the movable sheet feed rollers and "D" shaped rollers, just before the cassette has been attached, the sheet feed rollers may touch the top sheet, resulting in sheet trouble.
In the above structure, after the cassette is attached and the sheet is fed by the sheet feed rollers, when the sheet is fed to the feed mechanism, the sheet feed rollers always touch the sheet, resulting in affecting precision of sheet feeding by the sheet feed mechanism.